The present invention generally relates to methods and devices for detecting the presence or absence of high frequency signals in a system, and more specifically relates to a signal amplitude comparator.
There are numerous methods to detect the presence or absence of high frequency signals in a system. When the signal is primarily of alternating current, at frequencies where it is somewhat difficult to generate gain in a given semiconductor process, at a signal swing that is small compared to the Vt of the device and power is limited, the task becomes more difficult.
Dual differencing amplifiers have been applied to signal processing and comparison tasks with good success. However, at high frequencies, it is difficult to get good performance.
Square law operation of MOS transistors can generate good signal levels at high frequencies for signal detection. The presence of the direct current term as a result of product detection has an average value that can be constant over a wide range of frequencies. However, typical process variation makes it difficult to get a repeatable threshold value.
A general object of an embodiment of the present invention is to provide a signal amplitude comparator which is configured to generate an output that accurately determines presence of a signal with a repeatable amount of amplitude.
Another object of an embodiment of the present invention is to provide a differential input circuit that is configured to generate an output current that is a non-linear function of an input voltage, and is configured to produce even order harmonic distortion over an input voltage range of interest.
Briefly, and in accordance with at least one of the forgoing objects, an embodiment of the present invention provides a signal amplitude comparator which includes a first differential input circuit that is biased, is configured to receive an input voltage and is configured to generate a first output current that is a non-linear function of the input voltage, a second differential input circuit which is biased similarly to the first differential input circuit, is configured to receive a reference input voltage and is configured to generate a second output current that generally tracks process, temperature and supply variation, and a comparator which is connected to the first differential input circuit and the second differential input circuit and is configured to receive the first output current from the first differential input circuit and the second output current from the second differential input circuit. The comparator is configured to compare the first and second output currents and generate an output which indicates whether the input voltage exceeds a pre-determined threshold value.